1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as a copier, a laser printer, or a facsimile which uses an electrophotographic process, and in particular to an arrangement of a drive unit used to drive a photosensitive member which the image forming apparatus has, and how the drive unit is controlled.
2. Description of the Related Art
Some image forming apparatuses form images using electrophotographic technology that electrostatically attracts and holds a developer comprised of fine powder. When images are to be formed using electrophotographic technology, first, an electrostatic latent image is formed on a surface of a photosensitive drum or a photosensitive belt which is an image carrier, and the formed electrostatic latent image is developed using toner which is a developer to generate a visible toner image. Then, the toner image is transferred onto a transfer material by a transfer device, and the toner image on the transfer material is fixed on the transfer material using pressure, heat, or the like. A print is thus obtained.
Obtaining an image comprised of a plurality of colors (color image) using electrophotographic technology can be realized by using developers of a plurality of colors. In principle, developers of respective colors, yellow, magenta, and cyan are used, and a color image is formed through color mixing that superposes toner images of the respective colors on top of one another. It should be noted that when a color image is to be formed, a black developer as well as colored developers is used as the need arises.
Examples of image forming apparatuses that form color images in the above described manner include an image forming apparatus in which developing devices for respective colors in which developers are stored are placed side by side on a straight line or a curve, and which forms and develops electrostatic latent images corresponding to the developers of the respective colors. Methods to form a color image by superposing toner images of respective colors on top of one another on a transfer material such as paper or a plastic sheet include a direct transfer method and an intermediate transfer method.
To form a high-quality color image on a transfer material, the rotation accuracy of an image carrier on which toner images are to be formed is required to be improved when any of the image forming apparatuses and the transfer methods described above is used. Accordingly, for example, there has been proposed a drive unit that detects a reference position of an image carrier using a sensor, and detects the rotational speed of the image carrier using a rotational speed detection unit such as an encoder (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. H06-327278). This drive unit carries out feedforward control of rotational speed when rotation of the image carrier is started, and at this time, detects a reference position of the image carrier and obtains a rotational speed profile of the image carrier to generate a signal that cancels out fluctuations in rotational speed and output the same to a motor that drives the image carrier.
However, when fluctuations in the rotational speed of the image carrier include a fluctuation caused by harmonic noise (harmonic rotational speed fluctuation), response is too late if a harmonic noise countermeasure instruction (signal) is output to the motor that drives the image carrier, and as a result, fluctuations in the rotational speed of the image carrier cannot be canceled out. Accordingly, there is an approach that outputs a harmonic noise countermeasure instruction to an exposure unit having superior control responsiveness.
However, even when the control method that outputs a countermeasure instruction to cope with a harmonic rotational speed fluctuation to the exposure unit is used, a harmonic rotational speed fluctuation cannot be satisfactorily canceled out when there is a large temporal difference between a harmonic rotational speed fluctuation and a countermeasure instruction (signal). This is because in response to a low-frequency noise countermeasure instruction, real-time feedback control is carried out to cope with a fluctuation in the rotational speed of the motor that rotatively drives the image carrier. Namely, the rotational speed of the image carrier for which a countermeasure against low-frequency noise has been taken greatly varies from rotation to rotation, and the variation results in a large temporal difference between a harmonic rotational speed fluctuation and a countermeasure instruction (signal).